Television receiver



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Ralph 6'. Holmes Ol/MS DEGREES Patented Jan. 25, 1944 TELEVISION RECEIVER Ralph S. Holmes, Haddonfleld, N. J., asslgnor to Radio Corporation of America, a corporation of,

Delaware Application May 1, 1340, Serial No. 332,625

5 Claims. (01. via-7.5)

My invention relates to television receivers and particularly to receivers of the type including a cathode ray tube and its associated deflecting circuits which are held in'synchronism with the is a separating tube or circuit for removing thepicture signal and for passing only the synchronizing pulses to the deflecting circuits. In order to obtain proper separation, the signal applied to the separating tube should be greater'than a certain minimum amplitude. Therefore, itis the usual practice to amplify the video signal before it is applied to the separating tube. Since it is generally desirable to use a separating tube of the type which requires that the synchronizing pulses be applied to its control grid with positive polarity, the use of a second synchronizing amplifler tube maybe required solely to reverse polarity if conventional practice is followed.

It is an object of my invention to provide an improved television receiver of the abovedescribed type.

Another object of my invention is to provide an improved means for and method of supplying a composite signal comprising picture signals and synchronizing signals to a separating tube or the taken on the output circuit of a video amplifier having gain control, rather than off some preced-' ing point such as the output circuit 01 the second detector. It follows that whenever the gain of the video amplifier is varied to change the picture contrast, there is a change in the amplitude of the synchronizing pulses supplied to the separating' tube. This is undesirable since a separating amplitude range. I overcome the above undesirable picture by providing thesynchronizing amplifier with a suittube operates best on signals within a certain able gain control which is ganged with the video amplifier gain control in such a way that the synchronizing amplifier output remains substantially constant even though thegain of the video amplifier is varied.

"rne inventionwill be better'understood from the following description taken in connection with the accompanying drawing in which trast contro Figure 1 is a circuit diagram of a preferred embodiment of my invention,

Eigure 2 is a circuit diagram of a difierent embodiment of my invention, and

Figure 3 is a pair of curves showing the characteristics of the gain control potentiometers which are shown ganged in Figure 1.

Referring to Fig. 1 of the drawing, there is shown a portion of a television. receiver designed for the reception of a television signal, such, for example, as acarrier wave which is negatively modulated by a composite signal consisting of picture signals and synchronizing pulses where the synchronizing pulses extend beyond black (i. e., to blacker-than-blacki) and therefore are of greater amplitude than the picture signals. The synchronizing signalsrinclude horizontal synchro nizing signals, one of which occurs at the end of each scanning line, and vertical synchronizing or framing pulses of the same amplitude as, but of greater duration thani a horizontal synchronizing pulse, a framing pulse occurring at the end of each vertical deflection.

The receiver may be of the superheterodyne type comprisinga second detector l0 which supplies the above-described composite signal consisting of picture signal and synchronizing pulses to a suitable cathode ray tube I through a single video amplifier stage l2. Since the synchronizing pulses should be impressed upon the control elecnected between the cathode I8 of the pentode and ground, this resistor having no bypass con-' denser whereby there is a certain amount of degeneration, depending upon the value of the resistor. Therefore, a change in the resistance value of the cathode resistor changes the'gain'of the video-amplifier l2. Such again control in a television receiver is also referred to as a conlhe receiver also includes the usual horizontal and vertical deflecting circuits l9 and 2| which are synchronized bythe incoming synchronizing pulses for the purpose of keeping the scanning at circuits I9 and 2 l, the composite signal consisting of picture signals and synchronizing pulses is taken oiI the output circuit of the video amplifier 12 even though this amplifier contains a gain or contrast control and supplied through a suitable synchronizing amplifier 22 to a separating tube 23 or separating circuit which passes only the synchronizing pulses to a filter circuit 24. Suitable filter circuits are well known in the art and preferably comprise resistor and capacitor networks so arranged that the horizontal and vertical deflecting pulses are separated from each other.

In th example illustrated, the synchronizing amplifier 22 is a triode having in its input circuit a grid condenser 26 and a'grid resistor 21 of such capacity and resistance values that the tube is grid-leak biased. It will be noted that the picnecessary design information for the gain control resistors l1 and 28 for the case where the particular amplifier tubes of the types indicated in Fig. 3 are employed. It will be noted that in this specific example these curves are for a design where the resistance of the cathode resistors l1 and 28 is varied by rotating an arm in contact with resistance material, the arms of the two variable resistors being connected to a rotatable shaft which may be rotated by means of a contrast control knob on the panel of the receiver.

In Fig. 2 there is illustrated another embodiment of my invention which is the same in principle as the embodiment above described. In this embodiment, however, the design is simplified by connecting the cathode of the video amplifier 38 and synchronizing amplifier 31 together by means of a cathode resistor 38 which has a variable contact point 33 thereon through which conture signals, since they are of positive polarity at this point in the circuit, drive the grid positive to provide the bias. This, of course, results in picture signal distortion but since the picture signals are to be removed by the synchronizing separator tube anyhow, this is immaterial. It

' will be understood that the amplifier 22 may be biased in any other suitable manner, for example, in the same. way as the video amplifier l2.

Since, with the connection above described, any

fier will change the amplitude of the synchronizing pulses supplied to the synchronizing amplifier, and, since it is ver -desirable that there be no great variation in the amplitude of the synchronizing pulses when applied to the synchronizing separator 23, the synchronizing amplifier 22 is provided with a gain control which, in the example illustrated, consists of a variable cathode resistor 28 connected between the cathode 29 oithe triode and ground. The variable resistor 23 and the variable gain control resistor l1 0! the video amplifier are ganged or mechanically coupled together in such a way that, when the cathode resistor ll of the video amplifier is changed to increase the gain of this amplifier,

the resistance of the other cathode resistor 28 change in the contrast control of the video amplibiased type described and claimed in application Serial No. 23,136 filed May 24, 1925, in the name of William A. Tolson, and assigned to Radio Corporation of America. It will be noted that this preferred type of separating tube requires that the synchronizing pulses be applied to its grid with positive polarity. It will be apparent, therefore, that, if the usual practice were followed in the receiver illustrated, the synchronizing signalsdor the synchronizing channel would be taken oi! the second detector ill and a second synchronizing amplifier stage would be required in order to reverse the polarity of the synchronizing pulses.

In Fig. 3, curves are shown which give the nection is made to ground.

In this arrangement, as in the one previously described, the gain of the video amplifier 33 and of the synchronizing amplifier 31 is controlled by means of degeneration. It will be apparent from an inspection of Fig. 2 that it the variable tap 39 is moved in a direction to-increase the resistance of the cathode resistor for the video amplifier 36 and thereby decrease the gain of the amplifier, the cathode resistance for the synehro nizing amplifier 31 will simultaneously be decreased thereby simultaneously increasing the gain of the synchronizing amplifier.

It will be apparent that the circuit of Fig. 2 operates similar to the circuitof Fig. 1 to maintain the amplitude of the signal output of the synchronizing amplifier substantially constant regardless of changes made in the gain of the video amplifier. It may be noted that in eneral the embodiment shown in Fig. 1 is preferred, since it permits greater flexibility in design. In receiver designs where low cost is a more important consideration, however, the design illustrated in Fig. 2 may be preferred.

While I have shown and described a degenerative type of circuit for controlling the gain of the video and synchronizing amplifiers, it should be understood that other well-known gain control circuits may be employed instead.

I claim as my invention:

1. In a television receiver comprising .a cathode ray tube and associated deflecting circuits for deflecting the cathoderay, means for demodulating a carrier wave which is modulated by picture signals and synchronizing pulses, a

video amplifier connected between said demodulating means and said cathode ray tube, said video amplifier comprising a stage having gain control means, a synchronizing amplifier stage; means for separating picture signal from synchronizing pulses, said synchronizing amplifier stage and said separatingmeans being connected in the order named between said video amplily constant regardless of changes in the gain of said video amplifier.

2. In a television receiver comprising a cathode ray tube and associated deflecting circuits for defiectingthe cathode ray, means for demodulating a canier wave which is modulated aaeasao by picture signals and synchronizing pulses, av video amplifier connected between said demodulating means and said cathode ray tube, said video amplifier comprising a stage having gain control means, a synchronizing amplifier stage, said synchronizing amplifier stage being connected between said video amplifier and said deflecting circuits, the synchronizing amplifier being'connected to said video amplifier at a point following said gain control means, and means for changing the gain of the synchronizing amplifier in one sense in response to changing the gain of the video amplifier in the opposite sense.

3. In a television receiver comprisinga cathode ray tube and associated deflecting circuits for deflecting the cathode ray, means for demodulating a carrier wave which is modulated by picture signals and synchronizing pulses, asingle video amplifier stage so connected between said demodulating means and said cathode ray tube as to have synchronizingpulses of negative polarity in its output, said amplifier stage having gain control means, a single synchronizing amplifier stage, a grid-leak-biased tube for separating picture signal from synchronizing pulses,

. said synchronizing amplifier stage and said separating tube being-connected in the order named between the output circuit of said video amplifier stage and said deflecting circuits,.gain control means for said synchronizing amplifier stage, and means for so gauging said two gain control means that changing the gain of said video amplifier in one sense causes a change in gain in the synchronizing amplifier in the opposite sense.

4. In a television receiver comprising a cathode ray tube and associated deflecting circuits for deflecting the cathode ray, means for demodulating a carrier wave which is modulated by picture signals and synchronizing pulses, av

plifier in one sense and changes the gain in the single video amplifier stage so connected between said demodulating means and said cathode ray tube as to have synchronizing pulses of negative polarity in its output, said amplifier stage having gain control means consisting of a variable cathode resistor, a single synchronizing amplifier stage, means for separating picture signal from synchronizing pulses, said synchronizing amplifier stage and said separating means being connected in the order named between the output circuit of said video amplifier stage and said deflecting circuits, said synchronizing amplifier stage having gain control means consisting of a variable cathode resistor, and means for so ganging said two gain control means that changing the gain of said video amplifier in one sense causes a change in gain in the synchronizing amplifier in the opposite sense.

5. In a television receiver comprising a cathode ray tube and associated deflecting circuits for deflecting the cathode ray, means for demodulating a carrier wave which is modulated by picture signals and synchronizing pulses, a single video amplifier tube so connected between said demodulating means and said cathode ray tube as to have synchronizing pulses of negasynchronizing amplifier in the opposite sense.

RALPH S HOLMES. 

